MRI technology utilizes an interaction between magnetic field and nuclear spins in order to form two-dimensional or three-dimensional images of patients for medical diagnosis and research. To perform a study the patient is positioned within an MRI scanner which forms a strong uniform magnetic field around the area to be imaged. By application of a magnetic field, the hydrogen atoms in the sample (usually originated in water molecules in the body soft tissues) are excited and emit a detectable RF signal using energy from an oscillating magnetic field applied at the appropriate resonant frequency. The distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse). In order to realize spatial resolution in the body, switching magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state. A set of signal data is then converted to an MR (magnetic resonance image).
Magnetic resonance imaging device (MRD) design is essentially determined by the type and format of the main magnet, i.e. closed, tunnel-type MRI or open MRI.
In general, three main types of magnets are used in MRI systems: resistive, permanent and superconducting. The most commonly used magnets are superconducting electromagnets. These consist of a coil that has been made superconductive by helium liquid cooling, and further maintained cold by a cryocooler, refrigerator, or liquid nitrogen. Superconducting electromagnets produce strong, homogeneous magnetic fields, but are expensive and require regular maintenance.
Additionally, three gradient MRI magnets are used in the MRI machine to help the imaging process. They create a variable field after the other magnets have been activated to generate a stable field, and are turned on and off very quickly to create different pictures or “slices” for spatial distribution of the image, and for enabling analysis of the rate of magnetic decay and recovery between different magnetic pulses, thereby receiving a more thorough and in-depth examination of the patient.
Low field MRI also uses resistive electromagnets, able to create a magnetic field when electricity runs through them. They are cheaper and easier to maintain than superconducting magnets. Resistive electromagnets are far less powerful, use more energy and require a cooling system. Other magnets in use are permanent magnets, of different formats, that are composed of ferromagnetic metallic components. Although they have the advantage of being inexpensive and easy to maintain, they are very heavy and weak in intensity.
Maintaining a uniform and stable magnetic field is a necessity for producing quality imaging. Among the factors affecting this field are temperature, electromagnetic interference, and movement. When referring to small magnetic resonance devices (MRD) the magnet used is usually a permanent magnet. Since a permanent magnet does not produce heat, and being rather small in size the device's heat retention capabilities are low, the MRD is exposed to the environmental temperature. Further, being small in size, thus transportable, requires that the MRD will be able to work in different environmental conditions.
Having a large signal to noise ratio (SNR), the MRD needs to be as un-interrupted as possible during the examination. EMI (electro magnetic interference) generated at an external source such as from electric lines, television and radio signals, elevators, etc., can impede MRI operation and analysis. Even small electrical circuits like a conversion circuit of the DC power source in a computer, can generate electromagnetic interference. EMI can result in serious tampering of the magnetic field uniformity, and impairment of the generated RF signals, therefore resulting in either artifacts or missing information.
Facilities providing MRI services build specially designed rooms that allow MRI procedures to be shielded from these interferences, while preventing leakage of the same interferences to the outside.
This shielding may include passive or active components to achieve magnetic and RF shielding. For example, to achieve RF shielding, the walls, floor and ceiling are built from sheets of conductive metal such as copper, aluminum, etc., including a door that maintains a closed circuit with the walls. Magnetic shielding could be provided by constructing a magnetic shield around the RF shield. A passive solution involves using magnetic shielding material, typically metal or metal alloy. These materials would need to be comprised of a very high permeability material such as “mu-metal”. The second option would be an active magnetic cancellation system, that would typically include a magnetometer, controller, amplifier and compensation coils. This solution tends to be costly and requires adjusting and handling.
Obtaining best results from an MRI scan, and thereby increasing the efficiency of the imaging process requires homogeneity of conditions. Homogeneity of the MRI scanning conditions also enables reliable comparison of MRI scans taken at different times from the same individual or sample, allowing for better monitoring of small changes. Other environmental factors like temperature can also affect the uniformity of the magnetic field, by affecting the properties of the magnetic alloys, and by affecting the properties of a scanned sample.
Therefore, there is a need for an RF shielding jacket providing shielding of the MRD from the external environment electromagnetic interference, thereby allowing for homogenized imaging conditions. The present invention further provides an RF shielding jacket combined with passive temperature insulating properties, fitting for an MRD.